An amplifier is a device that receives an input signal (e.g., a voltage or a current) and generates an output signal based thereon. For example, a differential amplifier, which is a common type of amplifier, receives positive and negative versions of an input signal (i.e., a differential input signal) and produces an amplified output signal based on the voltage difference between the two input versions.
Conventionally, a bootstrap circuit including various electronic components (e.g., resistors or transistors) may be used to improve the linearity of an amplifier by preventing a logarithmic change in a base-emitter voltage caused by a change in the collector-base voltage of an input transistor of the amplifier. The unity gain bootstrap circuit implemented around the input transistor in this example ties its collector voltage to its base, thereby keeping the base-collector junction at a constant voltage; as a result, the collector current and the common-emitter current gain (i.e., β) remain constant at all operational input voltages.
Phase inversion refers to the output voltage changing in a “wrong” direction with respect to a change in the input voltage; an amplifier may experience phase inversion when, for example, the input voltage changes too quickly for the bootstrap to respond. The collector-base junction of an NPN bipolar junction transistor (BJT) is reversed biased in a forward-active region; upon receiving a large and/or fast voltage change, the voltage at the base may be larger than the voltage of the collector that has not yet responded. This condition results in saturation of the NPN BJT, which, in turn, may cause the output voltage to temporarily change in the wrong direction (i.e., phase inversion occurs). This problem is worse when the input signal varies at a high frequency and/or when the input step has a low rise-time.
Consequently, there is a need for circuitry that can economically improve the linearity of an amplifier (via, e.g., a bootstrap circuit) but prevent phase inversion upon receiving a large, fast change in the input signal.